The first observation of electroluminescence in organic materials was in the early 1950s by Andre Bernanose and his co-workers at Nancy-University in France. In 1963, Martin Pope and his co-workers at New York University first observed direct current (DC) electroluminescence on a single pure crystal of anthracene and on anthracene crystals doped with tetracene under vacuum. The first diode device was created by Ching W. Tang and Steven Van Slyke at Eastman Kodak in 1987. The diode device used a two-layer structure with separate hole transporting and electron transporting layers, resulting in reduction of operating voltage and improvement of the efficiency, thereby leading to the current era of organic EL research and device production.
Typically, organic EL device is composed of organic material layers sandwiched between two electrodes. The organic material layers include the hole transporting layer, the light emitting layer, and the electron transporting layer. The basic mechanism of organic EL involves the injection, transport, and recombination of carriers as well as exciton formation for emitting light. When an external voltage is applied across the organic EL device, electrons and holes are injected from the cathode and the anode, respectively. Subsequently, the electrons recombine with holes in the light emitting layer to form excitons and then emit light. The structure design of the organic material layers and the selection of anode and cathode are critical for the OLED device to fully exhibit luminous efficacy.
In subsequent studies, it was found that when the light emitting layer is sandwiched between Alq3 and NPB, OLED can be improved by doping the light emitting layer with a dopant such that the light energy of the host can be transferred to the dopant for changing the color of light. Therefore, the red, blue, and green light emitting OLED devices could be obtained, which makes OLED advance greatly toward the full-color display. However, in such a situation, the researchers have to consider the physical properties of the materials themselves, such as the energy level difference, thermal properties, morphology, etc. so as to find the optimum materials. The researchers need to do the research and improvement repeatedly in order to meet the requirements.
Organic EL is applied to flat panel displays due to their high illumination, low weight, ultra-thin profile, self-illumination without backlight, low power consumption, wide viewing angle, high contrast, simple fabrication processes and rapid response time. In OLED development, a full-color flat-panel display is the highest goal in development. At present, the red, blue and green doping materials have been successfully developed, but have not yet reached a satisfactory level. There still exists a need to continue research and development of new and better doping materials. In addition, white light OLED is also a recent research focus and expected to be used as a lighting source or LCD screen backlight so as to significantly reduce the volume and weight of the display device using the conventional white light source.
Recently, a new type of fluorescent organic EL device has been developed by Adachi and coworkers. The new organic EL device incorporates the mechanism of thermally activated delayed fluorescence (TADF), which is a promising way to obtain a high efficiency of exciton formation by converting spin-forbidden triplet excitons up to the singlet level through the mechanism of reverse intersystem crossing (RISC). However, there still exists a need for the compound with TADF property and high luminous efficiency.